Semi-submersible offshore platforms are frequently used when drilling, producing or storing hydrocarbons, such as oil and gas, at sea. They are best known for their ability to withstand the environmental forces subjected to the platform by the wind and the sea, primarily in terms of movements and independency of direction of the environmental forces.
Conventional semi-submersible offshore platforms are used primarily in offshore locations where the water depth exceeds about 90 m. This type of platform comprises a hull structure that has sufficient buoyancy to support the equipment deck above the surface of the water. The hull typically comprises one or more submersible pontoons that support a plurality of vertically upstanding columns, which in turn support the deck above the surface of the water. The size of the pontoons and the number of columns are governed by the size and weight of the deck and equipment being supported.
One example of such a semi-submersible offshore platform is described in the patent publication of GB 2,310,634. The semi-submersible platform for storing liquid hydrocarbons comprises a superstructure and six spaced apart legs extending from the superstructure. The superstructure can be equipped with buildings and drilling or production equipment. Each of the legs is divided by an internal wall which defines a storage tank spaced radially inwardly from each of the respective leg. The legs are rigidly interconnected at end portions thereof which are disposed remote from the superstructure by a ring pontoon. Likewise, in the patent publication with the U.S. Pat. No. 4,498,412 is a semi-submersible offshore platform described. The platform comprises an operating deck carried by four cylindrical columns supported by a pontoon structure comprising four sided boxes formed into a square ring.
The above mentioned platforms each utilize the well established technique of using a plurality of columns to minimize the effect of the environmental forces as well as obtaining an appropriate stability of the platform. The wind and the sea can pass underneath the operating deck while the plurality of columns imposes stability to the operating deck by providing several support points to the sea. However, this advantage comes with the price of subjecting pipes and drilling equipment, which extends between the operating deck and the sea floor, to the same environmental forces.
This drawback has been at least partly solved by the platform described in the publication of U.S. Pat. No. 6,945,736 B2. The platform is designed mainly as a vertical flat bottomed cylinder and comprises a centrally arranged vertical through shaft, also referred to as a moonpool, for receiving of risers or other drilling equipment. The cylinder wall comprises a number of tanks in which liquid can be stored. However as the need for offshore solutions increases, the need for platform bodies which are capable of taking on a wide variety of facilities and equipment are needed. When larger facilities, e.g. production facilities, are positioned on platform bodies, there is a constant need to maintain the point of balance so that any facility not risk of shifting the point of balance in an unwanted or unexpected direction. Usually these shifts can be contravened by moving ballast between ballast tanks to compensate of the diversions in point of balance. When storing huge quantities of hydrocarbons this compensation however provides losses in storage capacity. This is indeed a deficiency of known solutions.